Question: Nadia is 4 times as old as Christopher and is also 30 years older than Christopher. How old is Christopher?
Solution: We can use the given information to write down two equations that describe the ages of Nadia and Christopher. Let Nadia's current age be $n$ and Christopher's current age be $c$ $n = 4c$ $n = c + 30$ Now we have two independent equations, and we can solve for our two unknowns. Since we are looking for $c$ , and both of our equations have $n$ alone on one side, this is a convenient time to use elimination. Subtracting the second equation from the first equation, we get: $0 =$ $4c$ $-$ $ (c + 30)$ which combines the information about $c$ from both of our original equations. Solving for $c$ , we get: $3 c = 30$ $c = 10$.